Dual polarized cylindrical reflector antenna system

ABSTRACT

A dual polarization antenna system comprising a cylindrical reflector having a parabolic cross section. The cylindrical reflector simultaneously collimates energy directed from a vertically polarized and a horizontally polarized line source, both of which are offset from the cylindrical reflector to avoid aperture blockage. A folded pill box may be used for each of the line sources.

J l J i I J U Q J! 0 United States Patent 1191 1111 3,810,185 Wilkinson May 7, 1974 [54] DUAL POLARIZED CYLINDRICAL 2,912,694 11/1959 Phillips 343/756 REFLECTOR ANTENNA SYSTEM 2,983,918 5/1961 Parmeggiani 343/756 3,029,43l 4/1962 Miller 343/756 [75] Inventor: Ernest J. Wilkinson, Sudbury, Mass.

[73] Assignee: Communications Satellite Primary Examiner-13 Lieberman Corporation, Washington DC. Attorney, Agent, or FirmAlan J. Kasper; Martln C.

Fl 1 221 Filed: May 26, 1972 er [21] Appl. No.: 257,094 [57] ABSTRACT A dual polarization antenna system comprising a cy-. [52 us. Cl 343/756, 343/779, 343/786 lindrical reflector having a parabolic Cross Section- 51 Int. Cl. H0ls 19/00 The cylindrical reflector simultaneously collimates [58] Field of Search 343/756, 779, 786 y directed from a vertically Polarized and a horizontally polarized line source, both of which are offset [56] R f n Cit d from the cylindrical reflector to avoid aperture block- UNITED STATES PATENTS age. A folded pill box may be used for each of the line 2,790,l69 4/]957 'Sichak 343/756 Sources 2,767,396 l0/l956 Cutler 343/780 16 Claims, 11 Drawing Figures mmmwv 7 i914 SHEET 1 0F 6 VERTICALLY POLARIZED ENERGY 29 F0c/:L LINE FOR BACK SURFACE 3 FOCAL LINE FOR FRONT SURFACE 5 FIG.1

PATENTEDHAY 9 sum 2 (If 6 amgmium '1 1974 13,810,185

SHKEI .3 0f 6 BEAM DIRECTION BEAM DIRECTION FOC L AXES VERTEX FIGH ZMENTEUHAY 7 1974 SHEU 5 (if 6 N. at

BACKGROUND OF THE INVENTION The present invention relates to an antenna system and, more particularly, to an antenna system having dual polarization capabilities for use in a satellite communications system.

The role of satellite communications in meeting the demand for communications services is becoming increasingly important. In order for satellite communications systems to meet the demand of more users and the demand for more types of use, e.g. data, voice, TV, facsimile, the capacity of the satellite communications system to accommodate such demand must increase.

The capacity of present day satellite communications systems is limited due, in part, to bandwidth constraints. Presently, satellite communications systems operate in the 4-6 Ghz range and are allocated a frequency spectrum of 500 MHz. The 500 MHz spectrum may be divided, for example, amongst 12 transponders on the satellite wherein each transponder may operate over a 40 MHz band. The remaining MHz of bandwidth may be allocated for guard bands which lie between each of the 40 MHz channels, as is known. There is, therefore, a real limit to the number of channels available to earth stations in the system due to such limited spectrum allocation.

One technique for increasing the capacity of such satellite systems is known as a dual polarization technique. Dual polarization enables an earth station to transmit two independent channels of information via coincident a'ntenna beams on identical carriers. Dual polarization may be accomplished by generating two antenna beams which are either linearly polarized, i.e., orthogonal to each other, or circularly polarized, i.e., one beam is rotating clockwise while the other beam is rotating counterclockwise. There is, therefore, an increase in available spetrum by means of frequency reuse. Since two independent channels of information may be transmitted over coincident antenna beams on identical frequencies, dual polarization provides a doubling of the satellite communications capacity.

The quality of any system employing a dual polarization technique is dependent upon an antenna system having low cross polarization levels. That is, since the coincident polarized antenna beams are carrying two channels of information on identical frequencies, the only isolation between the two channels is that obtainable by an antenna system which reduces the response of each polarized beam to orthogonally polarized radiation. In addition, it is also desirable for such dual polarization antenna systems to maintain excellent gainbeamwidth product.

All satellite communications systems today employ reflector type antennas which are known as paraboloids of revolution (i.e. the well-known. dish type). These dish" antennas, which are rotationally symmetric, have a circular beam shape and are, therefore, useful for covering areas of circular cross section. Such dish" antennas can be designed to have relatively low cross polarization levels and an excellent gainbeamwidth product; however, if an elliptically shaped satellite antenna beam is required in a dual polarization system, then both low cross polarization levels and excellent gain-beamwidth products are not easily obtained simultaneously in these antennas. Such elliptically shaped beams would be useful where an elliptically shaped area, such as the continental United States, is to be covered.

Rotationally symmetric paraboloids of revolution having a circular beam shape can be designed to exhibit low cross polarization levels over their beam coverage area and excellent 'gain-beamwidth product provided the feed pattern is also rotationally symmetric with respect to the focalaxis and aperture blockage is minimized e.g. by minimizing dimensions of the feed supporting struts and the size of the feed. However, if a paraboloid of revolution is elliptically shaped (thereby providing an elliptically shaped antenna beam) instead of circularly shaped, the cross polarization level will rise unless the gain-beamwidth product is deliberately sacrificed. A sacrifice of the gain-beamwidth product is not desirable, though, for communications satellite systems. Also, if the feed for the paraboloid of revolution (elliptically or circularly shaped) is offset to avoid intercepting rays coming off the reflector, i.e., to eliminate aperture blockage, the cross polarization level will rise. In this latter case, an offset feed will, in one respect, lower the cross polarization level because no energy reflected by the reflector will strike the offset feed to thereby produce cross polarization components; however, the overall cross polarization level of such antenna systems will rise, particularly near the edge of the beam coverage area, due to the effect of making the paraboloid asymetric in shape.

The prior art does not disclosean antenna system employing an offset feed technique to avoid aperture blockage and an elliptically shaped reflector to provide an elliptical beam shape while maintaining sufficiently low cross polarization levels and sufficiently high gainbeamwidth productfor purposes of frequency reuse. The present invention does provide an offset feed antenna system having extremely low cross polarization levels, relatively high, gain-beamwidth product and elliptically shaped, dual polarized coincident beams, simultaneously, in a single system. These and other advantages of the present invention will be described below.

SUMMARY OF THE INVENTION In accordance with this invention a cylindrical reflector having a parabolic cross section, Le, a parabolic cylinder, is used as the main reflecting surface of the antenna system. In one embodiment the main cylindrical reflecting surface actually comprises two surfaces which individually reflect orthogonally polarized energy. A solid cylindrical back surface is used to reflect energy of a first type of polarization (e.g., horizontally polarized'energy) whereas a front cylindrical surface comprising a grating structure is designed to be transparent to energy of the first type of energy (i.e. horizontally polarized energy), but which reflects energy of a second type of polarization (i.e. vertically polarized energy). This dual, cylindrical reflector reflects energy transmitted from two pill box type line source feeds wherein each pill box generates, respectively, horizontally and vertically polarized energy. The pill boxes are positioned at the focal lines of the respective back and front surfaces of the dual, cylindrical reflector and are off-set from the respective reflecting surfaces to avoid aperture blockage.

In a second embodiment, the cylindrical reflector of the present system comprises a single, solid reflector which reflects both horizontally and vertically polarized energy. Two pill boxes are positioned on opposite sides ofa grating which reflects energy of one polarization and is transparent to energy of the second polarization. On pill box is located at the focal line of the cylindrical reflector and transmits its energy directly to the reflector through the grating. The other pill box is located above the grating and directs its energy onto the grating which then reflects such energy onto the cylindrical reflector. The latter pill box is positioned at the virtual focal line of the cylindrical reflector, i.e., the grating acts simply as a mirror for the polarized energy from the latter pill box. Again, both pill boxes are offset such that there is no aperture blockage.

The respective pill boxes which generate horizontally and vertically polarized energy are folded to minimize blockage of energy propagating in the pill boxes by the point source feed horn of the respective pill boxes. The pill box which excites horizontally polarized energy includes a grating located in its aperture and short circuits located at its coupling slot to eliminate vertically polarized energy which is unavoidably excited therein. In the alternative, the vertically polarized pill box comprises a multimode aperture pill box.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a view, in perspective, of the antenna system in accordance with one embodiment of the present invention.

FIG. 2 is a view, in perspective, of an alternative embodiment of the antenna system of the present invention.

FIG. 3 is a side view of the apparatus of FIG. 1.

FIG. 4 is a side view of the apparatus of FIG. 2.

FIG. 5A is a side side of a pill box used in the present invention.

FIG. 5B shows a cross section of the pill box of FIG. 5A.

FIG. 5C shows another cross section of the pill box of FIG. 5A.

FIG. 6 is a side view of an alternative form of pill box having a multi-mode aperture useful in the present in vention.

FIGS. 7, 8A and 8B shows some of the characteristics of the antenna system of the present invention.

DETAILED DESCRIPTION OF THE INVENTION Referring to FIG. 1, there is shown an antenna system in accordance with one embodiment of the present invention. Shown therein is a cylindrical reflector I having a parabolic cross section (i.e., a parabolic cylinder). The surface of the parabolic cylinder is generated by a parabolic element which is moved parallel to a fixed straight line. An example of a parabolic cylinder is shown in Antennas, by Lamont Blake, Chap. 6, John- Wiley & Sons, I966.

Cylindrical reflector 1 comprises a cylindrical, back, reflecting surface 3 and a cylindrical, front, reflecting surface 5. The cylindrical front, reflecting surface 5 is formed by a series of spaced, thin, parallel, metallic plates 7 which function as a polarized grating. The series of plates 7 are connected to the back, reflecting surface 3 and extend forward therefrom a predetermined distance to provide the cylindrical, front, reflecting surface 5. The spacing or width between each parallel plate 7 and the height of each parallel plate 7 is designated such that the polarized grating comprising the series of parallel plates 7 can propagate (i.e. act as a waveguide) for one type of polarized energy (e.g. horizontal) while acting as a reflector for orthogonally (vertically) polarized energy, as would be known by one skilled in the art.

Polarized energy is directed to cylindrical reflector 1 by two pill box" feeds 9 and 11. As will be described below, pill boxes 9 and 11 are designed to excite, propagate and direct, respectively, vertically and horizontally polarized energy towards cylindrical reflector 1. Pill box 9 comprises point source horn feed 13, a top waveguide 15 connected to point source horn feed 13, a cylindrical reflecting plate 17 having a parabolic cross section and a lower waveguide 19 terminating in a flared aperture 21. Top waveguide 15 is separated from lower waveguide 19 by means of a plate 23 which terminates a predetermined distance from reflecting plate 17 to provide a coupling slot 25. Pill box 9 is shown as having the top plate of top waveguide 15 removed to indicate the manner in which energy propagates through the pill box to flared aperture 21. The structure of horizontally polarized pill box 11 is similar to vertically polarized pill box 9 except that the former includes two short circuits across its coupling slot (not shown in FIG. I) and a polarization grating 27. Both pill boxes will be described in more detail in connection with FIGS. 5A, 5B and 5C.

As shown in FIG. 1, vertically polarized energy 29, i.e. energy which is polarized in a plane parallel to the grating formed by parallel plates 7, is directed from flated aperture 21 to cylindrical reflector 1. As mentioned previously, since the polarized grating formed by parallel plates 7 is designated to act as a waveguide only to horizontally polarized energy, the vertically polarized energy 29 will be reflected at the front surface 5 of cylindrical reflector l. Horizontally polarized energy 31, i.e. energy which is polarized in a plane perpendicular to parallel plates 7, is directed in a similar manner from the flared aperture of pill box 11 to cylindrical reflector 1. This energy will propagate through the waveguide formed between the parallel plates 7 to back, reflecting surface 3 which will reflect such horizontally polarized energy 31. Proper collimation of the vertically and horizontally polarized energy by cylindrical reflector l is accomplished by positioning the line sources, i.e., the flared apertures of the respective pill boxes 9 and 11, at the focal lines of the respective reflecting surfaces 5 and 3.

Referring to FIG. 3, there is shown a side view of the antenna system of FIG. 1. Like numerals of FIG. I are used for the description of FIG. 3. Pill box 9 is positioned at the focal line of front reflecting surface 5 which is designed so as not to extend to its vertex, as shown. Pill box 11 is positioned at the focal line of back reflecting surface 3 which also does not extend to its vertex, as shown. In this manner, energy directed from pill box 9 may be collimated by front reflecting surface 5 in a plane parallel to the focal axis of surface 5 without any aperture blockage by pill box 9, as shown by the energy rays. In a like manner, aperture blockage of the energy from pill box 11 is also eliminated.

Cylindrical reflectors such as reflector 1 will generate some cross polarization components when the wavefront at the reflecting surface is not cylindrical. A noncylindrical wavefront will occur when a line source (pill box 9 or 11) which produces a cylindrical wavefront is so far away from the cylindrical reflecting surface that the line source itself begins to look like a point source which produces spherical wavefront. This occurs due to the spreading of the wavefront as it is propagated from, e.g., pill box 9 to its cylindrical reflecting surface 5. Consequently, one technique for minimizing such spreading is to make the focal length of cylindrical reflector l as short as possible relative to the length of the pill box line source. Another technique for minimizing such spreading is to provide the antenna system of FIG. 1 with two side plates (not shown) which will guide the energy and reduce the spreading of the wavefront as it propagates from the pill boxes 9 and 11 to cylindrical reflector 1. The side plates would be positioned on each side of pill boxes 9 and 11 and cylindrical reflector l and would extend from the pill boxes 9 and 11 to cylindrical reflector 1.

Referring to FIG. 2 there is shown, in perspective, an

alternative embodiment of the antenna system of the present invention. Shown therein is a cylindrical reflector 33 of parabolic cross section, having a single, solid metallic reflecting surface 35. The cylindrical reflector 33 collimates energy received from vertically polarized pill box 37 and horizontally polarized pill box 39. Pill boxes 37 and 39 are the same as the respective pill boxes 9 and 11 described above. A flat, polarized grating 41 comprising a series of thin,-spaced, parallel, metallic plates 43 is positioned between vertically polarized pill box 37 and horizontally polarized pill box 39. The spacing or width between each parallel plate 43 and the length of parallel plates 43 are designed, in a known manner, whereby the polarized grating 41 may function as a waveguide for horizontally polarized enegy and as a reflector for vertically polarized energy.

Referring to FIG. 4, there is shown a side view of the antenna system of FIG. 2. Like numerals of FIG. 2 are used for the description of FIG. 4. Horizontally polarized pill box 39 which is positioned below the flat, polarized grating 41 directs its energy directly towards cylindrical reflector 33. The horizontally polarized energy, i.e., energy which is polarized in a plane perpendicular to the grating 41, propagates from pill box 39 through flat, polarized grating 41 to cylindrical reflecting surface 35 which collimates the energy. The line source or pill box 39 is positioned at the actual focal line of cylindrical reflector 33.

Vertically polarized pill box 37, which is positioned above flat, polarized grating 41, directs its energy towards the polarized grating 41. The vertically polarized energy, i.e. energy which is polarized in a plane parallel to grating 41, is reflected by grating 41 onto cylindrical, reflecting surface 35 which then collimates the energy. Vertically polarized pill box 37 is positioned at the virtual focal line of cylindrical reflector 33. Therefore, when vertically polarized energy from pill box 37 is reflected by polarized grating 41 onto cylindrical reflecting surface 35, that energy from pill box 37 appears to the cylindrical reflector 33 as if it is coming from the actual focal line. In this manner, the pill box 37 appears to the cylindrical reflector 33 as being physically positioned at its actual focal line, thereby enabling proper collimation of the energy from both pill boxes 37 and 39 by cylindrical reflector 33.

As described in connection with FIG. 1, the above two techniques for reducing the spreading of the cylindrical wavefront may be employed with this alternative embodiment. Also, as with the description of FIG. 3, cylindrical reflector 35 is designed so as not to extend to its vertex. Energy from pill boxes 37 and 39 will, therefore, be collimated by reflector 35 in a plane parallel to its focal axis and will not be blocked by either pill box.

Referring to FIG. 5A there is shown, in more detail, a side view of a horizontally or vertically polarized folded pill box which may be used in the embodiment of FIGS. I or 2. The pill box comprises a small point source horn feed 45 in which either vertically or horizontally polarized energy is excited. An upper waveguide 47, connected to horn feed 45, is defined by parallel plates 49 and 51. A lower waveguide 53, terminating in a flared aperture 55, is defined by parallel plates 51 and 57. Plate 51 terminates a predetermined distance from a cylindrical, parabolic reflector 59 to form a coupling slot 61.

Energy excited in point source horn feed 45, e.g. horizontally polarized energy, propagates from horn feed 45 through waveguide 47 to cylindrical reflecting plate 59 having a parabolic cross section. All the energy in waveguide 47 is then reflected by cylindrical reflector 59 and coupled to waveguide 53 by means of coupling slot 61. Coupling of all the energy from the waveguide 47 to waveguide 53 is accomplished by designing the coupling slot on the basis ofwaveguide'directional coupler theory with each ray of energy considered as a separate waveguide, as would be known. The collimated energy then propagates through waveguide 53 t0 flared aperture 55 having dimensions which provided for optimum illumination of cylindrical reflector l or 33 in the plane perpendicular to flared aperture 55. The use of a pill box comprising parallel plates which define an upper and lower waveguide, i.e. a folded pill box, eliminates blockage of the energy propagating in the pillbox by the point source horn feed. The dimensions of the waveguides 47 and 53 necessary to propagate either vertically or horizontally polarized energy and the design of the flared aperture 55 would be known to one skilled in the art.

FIG.'5B is a cross section along lines 1-1 of the pill box shown in FIG. 5A and shows, in addition thereto, two short circuits 63 and 65 for use only in horizontally polarized pill boxes. As is known, vertically polarized larized pill boxes such as pill boxes 11 and 39. Conversion of such horizontally polarized energy to vertically polarized energy would, if such energy is emitted at flared aperture 55, have the unwanted effect of increasing the cross polarization level of the antenna systems of FIGS. 1 and 2. Consequently, two short circuits 63 and 65, which connect plate 51 to cylindrical parabolic reflector 59, may be placed on opposite sides of, and equal distances from, the center of the coupling slot 61. By short circuiting the coupling slot 611 at these two points the amount of horizontally polarized energy which may be unavoidably converted to vertically polarized energy is reduced. Each short circuit 63 and 65 may comprise a thin wire.

FIG. 5C is a view taken along lines 2-2 of FIG. 5A and shows, in addition thereto, flared aperture 55 having a polarized grating 67 for use only with horizontally polarized pill boxes. Polarized grating 67 comprises a series of thin, spaced, parallel plates 69 extending across flared aperture 55. Polarized grating 67 may be used in conjunction with short circuits 63 and 65 to prevent any unavoidably excited vertically polarized energy which is not short circuited from being radiated by the horizontally polarized pill box. Such vertically polarized energy, which is in a plane parallel to the grating 67, would be reflected back into the horizontally polarized pill box.

Referring back to FIG. 4 again, relatively low cross polarization levels may be obtained with the use of vertically polarized pill box 37; however, as will be described, still lower cross polarization levels may be obtained if vertically polarized pill box 37 is replaced by a vertically polarized, multimode aperture pill box 71, shown in FIG. 6, which provides for low side lobe radiation. The multimode aperture, pill box 71 is similar to the pill box shown in FIG. 5A except that the former has two additional compartments 73 and 75 which support multimodes of energy. Vertically polarized energy of the TEM mode which is excited in point source horn feed 45 and which is coupled to waveguide 53 from waveguide 51, propagates along waveguide 53 to compartment 73. In compartment 73 some of the energy is converted to the TEm/TM mode, as indicated. Both the TEM and the TErz/TM 12 modes are then differentially phased while traveling through compartment 73 so as to produce an aperture distribution in compartment 75 yielding low side lobe radiation. The design of a multimode aperture pill box which can support multimodes of energy is well-known.

Low side lobe radiation reduces the cross polarization level in the following manner. Referring to FIG. 4, it can be seen that some energy, i.e. rays A through B, from the vertically polarized pill box, placed at the virtual focal line of reflector 35, will not be imaged onto reflector 35 from grating 43, but will be transmitted directly to reflector 35. Consequently, since this energy will not appear to reflector 35 as coming from its focal line, such energy will collimate into a pattern which peaks off axis on the lower side of the main beam pattern and will thereby cause an increase in the crosspolarization level. In addition, the summation of these two patterns results in a shift in the position of the maim beam. Since the increase in cross-polarization level and the amount of main beam shift is directly related to the side lobe level of the pill box itself, the multimode aperture pill box of FIG. 6, which reduces side lobe radiation, may be used in place of pill box 37. Finally, since a significant shift in the elevation beam position with frequency occurs due to the fact that the pill box side lobe radiation varies with frequency, the use of the multimode aperture pill box 71 will eliminate such beam shift.

In addition to the use of pill boxes described above, other forms of line sources, similarly protected by gratings from radiating cross polarization components, may be used. For example, a cylindrical lens or a phased array of dipole radiators may be used to replace the cylindrical reflector 59 of each pill box. Also, additional beams displaced from the focal axis direction may be formed by placing additional point source feed horns adjacent to the single horn 13 illustrated for example, in FIG. 1. The additional point source feed horns can be used for horizontally or vertically polarized pill boxes which use either a cylindrical reflector 59 or a lens collimator. For a dipole array a suitable multiple beam forming network, as is well-known, may be used for the same purposes. The additional beams may be used to cover, for example, Alaska or Hawaii, while the main, elliptical beam would be used for the continental US.

FIG. 7 shows the cross polarization characteristics of the antenna system of FIG. 1 over its full 3 dB beamwidth for both horizontal and vertical polarization. The 3 dB beam-width or contour for vertically polarized energy is shown as being bound by the dashed line while the 3 dB contour for horizontally polarized energy is shown as being bound by the solid line. The data shown in FIG. 7 is taken along the respective 3 dB contour lines which represent the worst case cross polarization values that can be found anywhere within the 3 dB contour. From FIG. 7 it can be seen that the worst case cross polarization level is still at least 43 dB below the on axis matched polarization level. Similar results were obtained with the antenna system of FIG. 2; however, the cross polarization levels were slightly poorer.

FIGS. 8A and 8B show, respectively, the gain, beamwidth and beam efficiency (which is proportional to the gain-beamwidth product) for vertical and horizontal polarization for the antenna system of FIG. 1. FIG. 8A shows the E and H plane beamwidths for vertical polarization and FIG. 8B shows E and H plane beamwidths for horizontal polarization wherein the E plane is defined, as is known, as the plane of polarization and the H plane is defined as the plane orthogonal to the plane of polarization. The characteristics shown in FIGS. 8A and 8B show that the antenna system of FIG. 1 can provide the low cross polarization characteristics of FIG. 7 while simultaneously providing excellent beam efficiency characteristics. The characteristics obtained with the antenna system of FIG. 2 were virtually identical to those shown in FIGS. 8A and 8B.

Finally, another advantage of the antenna system of the present invention is that the elliptically shaped cylindrical reflector l or 33 may be designed for any as pect ratio which is the beamwidth ratio in the principle planes while still maintaining all of the above described advantages.

What is claimed is:

1. An antenna system comprising a main cylindrical reflector having a parabolic cross-section and being adapted to radiate an elliptical-shaped beam originating from a point on the focal line of said reflector; first and second line sources, at least one of which is located at an actual focal line of said cylindrical reflector, wherein said first line source generates energy of one polarization and said second line source generates energy of a second polarization which is substantially orthogonal to said one polarization; and polarization grating means for propagating energy of said first polarization and reflecting energy of the second polarization whereby said antenna system is capable of generating simultaneously a dual polarized, elliptically shaped beam having low cross polarization and high gain.

2. The antenna system of claim 1 wherein said main cylindrical reflector comprises a first cylindrical reflecting surface having a parabolic cross section, and a second cylindrical reflecting surface, formed by said polarization grating, having a parabolic cross section, wherein said first cylindrical reflecting surface collimates energy from said first line source and said second cylindrical reflecting surface collimates energy from said second line source, and wherein said first and second line sources are, respectively, located at the focal line of the respective first and second cylindrical reflecting surfaces.

3. The antenna system of claim 2 wherein one line source is offset from said first cylindrical reflecting surface to substantially avoid aperture blockage.

4. The antenna system of claim 3 wherein said first and second line sources are folded pill box feeds, each said pill box comprising a first and second waveguide; energy exciting means, connected to said first waveguide, for exciting energy of one polarization; a cylindrical reflector means, having a parabolic cross section for reflecting said excited energy; a flared aperture, connected to said second waveguide; and coupling slot means for coupling said excited energy from said first waveguide to said second waveguide.

5. The antenna system of claim 4 wherein one of said pill boxes generates primarily horizontally polarized energy and wherein said horizontally polarized pill box further includes means for preventing vertically polarized energy which is excited therein from being radiated by said horizontally polarized pill box.

6. The antenna system of claim 1 wherein said main cylindrical reflector comprises a single reflecting surface; and wherein said polarization grating is located between said first and second line sources; and wherein said first line source is located at the actual focal line of said single reflecting surface and radiates energy directly towards said single reflecting surface through said grating; and wherein said second line source is located at the virtual focal line ,of said single reflecting surface and radiates substantially all of its energy towards said polarization grating.

7. The antenna system of claim 6 wherein one of said line sources is offset from said single reflecting surface to substantially eliminate aperture blockage.

8. A dual polarization antenna system comprising:

a. a main cylindrical reflector, having a parabolic cross section, comprising first and second cylindrical reflecting surfaces adapted to radiate an elliptical shaped beam originating from a point on the respective focal lines of said reflector surfaces;

b. a first line source means located at the actual focal line of said first cylindrical reflecting surface, for generating linearly polarized energy, and

c. a second line source means, located at the actual focal line of said second cylindrical reflecting surface, for generating linearly polarized energy which is substantially orthogonal to said first mentioned linearly polarized energy;

whereby said antenna system is capable of generating simultaneously a dual polarized, elliptically shaped beam having low cross polarization and high gain.

9. The antenna system of claim 8 wherein said first and second line source means are offset from said main cylindrical reflector to substantially eliminate aperture blockage.

10. The antenna system of claim 9 wherein each line source means comprises a folded pill box comprising:

a. means for exciting polarized energy;

b. a first waveguide means, connected to said means for exciting, for enabling the propagation of said polarized energy;

c. a second waveguide means for enabling the propagation of said polarized energy;

d. a cylindrical reflector means, having a parabolic cross section for reflecting said energy propagating in said first waveguide means;

e. coupling means for coupling said reflected energy from said first waveguide means to second waveguide means; and

f. a flared aperture connected to said second waveguide means.

11. The antenna system of claim 10 wherein one of said pill boxes primarily generates horizontally polarized energy and wherein said horizontally polarized line source includes means for preventing vertically polarized energy generated therein from being radiated by said horizontally polarized pill box.

12. The antenna system of claim 11 further comprising means for reducing the amount of spreading of the wavefront radiated by said pill boxes.

13. A dual polarization antenna system comprising:

a. a main cylindrical reflector having a parabolic cross section, comprising a single reflecting surface adapted to radiate an elliptical shaped beam originating from the point on the focal line of said surface;

b. a polarization grating means for propagating energy of a first linear polarization while reflecting energy ofa second polarization which is orthogonal to said first linearly polarized energy;

0. a first line source means, located at the actual focal line of said main cylindrical reflector, for radiating said first linearly polarized energy directly towards said main cylindrical reflector through polarization grating means;

d. a second line source means, located at the virtual focal line of said main cylindrical reflector for radiating substantially all of said orthogonally polarized energy towards said polarization grating means;

whereby said antenna system is capable of generating simultaneously a dual polarized, elliptically shaped beam having low cross polarization and high gain.

14. The antenna system of claim 13 wherein said first and second line source means are offset from said main cylindrical reflector to substantially eliminate aperture blockage.

15. The antenna system of claim 14 wherein each said line source means comprises a folded pill box comprising:

a. means for exciting polarized energy;

b. a first waveguide means, connected to said means for exciting, for enabling the propagation of said polarized energy; I

c. a second waveguide means for enabling the propagation of said polarized energy;

d. a cylindrical reflector means, having a parabolic cross section, for reflecting said energy propagating in said first waveguide means;

e. coupling means for coupling said reflected energy from said first waveguide means to second waveguide means; and

f. a flared aperture connected to said second waveguide means.

16. The antenna system of claim 15 wherein said second line source means comprises a multimode, flared 

1. An antenna system comprising a main cylindrical reflector having a parabolic cross-section and being adapted to radiate an elliptical-shaped beam originating from a point on the focal line of said reflector; first and second line sources, at least one of which is located at an actual focal line of said cylindrical reflector, wherein said first line source generates energy of one polarization and said second line source generates energy of a second polarization which is substantiAlly orthogonal to said one polarization; and polarization grating means for propagating energy of said first polarization and reflecting energy of the second polarization whereby said antenna system is capable of generating simultaneously a dual polarized, elliptically shaped beam having low cross polarization and high gain.
 2. The antenna system of claim 1 wherein said main cylindrical reflector comprises a first cylindrical reflecting surface having a parabolic cross section, and a second cylindrical reflecting surface, formed by said polarization grating, having a parabolic cross section, wherein said first cylindrical reflecting surface collimates energy from said first line source and said second cylindrical reflecting surface collimates energy from said second line source, and wherein said first and second line sources are, respectively, located at the focal line of the respective first and second cylindrical reflecting surfaces.
 3. The antenna system of claim 2 wherein one line source is offset from said first cylindrical reflecting surface to substantially avoid aperture blockage.
 4. The antenna system of claim 3 wherein said first and second line sources are folded pill box feeds, each said pill box comprising a first and second waveguide; energy exciting means, connected to said first waveguide, for exciting energy of one polarization; a cylindrical reflector means, having a parabolic cross section for reflecting said excited energy; a flared aperture, connected to said second waveguide; and coupling slot means for coupling said excited energy from said first waveguide to said second waveguide.
 5. The antenna system of claim 4 wherein one of said pill boxes generates primarily horizontally polarized energy and wherein said horizontally polarized pill box further includes means for preventing vertically polarized energy which is excited therein from being radiated by said horizontally polarized pill box.
 6. The antenna system of claim 1 wherein said main cylindrical reflector comprises a single reflecting surface; and wherein said polarization grating is located between said first and second line sources; and wherein said first line source is located at the actual focal line of said single reflecting surface and radiates energy directly towards said single reflecting surface through said grating; and wherein said second line source is located at the virtual focal line of said single reflecting surface and radiates substantially all of its energy towards said polarization grating.
 7. The antenna system of claim 6 wherein one of said line sources is offset from said single reflecting surface to substantially eliminate aperture blockage.
 8. A dual polarization antenna system comprising: a. a main cylindrical reflector, having a parabolic cross section, comprising first and second cylindrical reflecting surfaces adapted to radiate an elliptical shaped beam originating from a point on the respective focal lines of said reflector surfaces; b. a first line source means located at the actual focal line of said first cylindrical reflecting surface, for generating linearly polarized energy, and c. a second line source means, located at the actual focal line of said second cylindrical reflecting surface, for generating linearly polarized energy which is substantially orthogonal to said first mentioned linearly polarized energy; whereby said antenna system is capable of generating simultaneously a dual polarized, elliptically shaped beam having low cross polarization and high gain.
 9. The antenna system of claim 8 wherein said first and second line source means are offset from said main cylindrical reflector to substantially eliminate aperture blockage.
 10. The antenna system of claim 9 wherein each line source means comprises a folded pill box comprising: a. means for exciting polarized energy; b. a first waveguide means, connected to said means for exciting, for enabling the propagation of said polarized energy; C. a second waveguide means for enabling the propagation of said polarized energy; d. a cylindrical reflector means, having a parabolic cross section for reflecting said energy propagating in said first waveguide means; e. coupling means for coupling said reflected energy from said first waveguide means to second waveguide means; and f. a flared aperture connected to said second waveguide means.
 11. The antenna system of claim 10 wherein one of said pill boxes primarily generates horizontally polarized energy and wherein said horizontally polarized line source includes means for preventing vertically polarized energy generated therein from being radiated by said horizontally polarized pill box.
 12. The antenna system of claim 11 further comprising means for reducing the amount of spreading of the wavefront radiated by said pill boxes.
 13. A dual polarization antenna system comprising: a. a main cylindrical reflector having a parabolic cross section, comprising a single reflecting surface adapted to radiate an elliptical shaped beam originating from the point on the focal line of said surface; b. a polarization grating means for propagating energy of a first linear polarization while reflecting energy of a second polarization which is orthogonal to said first linearly polarized energy; c. a first line source means, located at the actual focal line of said main cylindrical reflector, for radiating said first linearly polarized energy directly towards said main cylindrical reflector through polarization grating means; d. a second line source means, located at the virtual focal line of said main cylindrical reflector for radiating substantially all of said orthogonally polarized energy towards said polarization grating means; whereby said antenna system is capable of generating simultaneously a dual polarized, elliptically shaped beam having low cross polarization and high gain.
 14. The antenna system of claim 13 wherein said first and second line source means are offset from said main cylindrical reflector to substantially eliminate aperture blockage.
 15. The antenna system of claim 14 wherein each said line source means comprises a folded pill box comprising: a. means for exciting polarized energy; b. a first waveguide means, connected to said means for exciting, for enabling the propagation of said polarized energy; c. a second waveguide means for enabling the propagation of said polarized energy; d. a cylindrical reflector means, having a parabolic cross section, for reflecting said energy propagating in said first waveguide means; e. coupling means for coupling said reflected energy from said first waveguide means to second waveguide means; and f. a flared aperture connected to said second waveguide means.
 16. The antenna system of claim 15 wherein said second line source means comprises a multimode, flared aperture. 